1. Field of the Invention
The present invention generally relates to the field of liquid crystal devices, and more specifically to a high speed reflective liquid crystal display and fabrication method in which electrode driver devices are integrally formed in a transparent monocrystalline silicon semiconductor layer.
2. Description of the Related Art
A typical reflective liquid crystal display includes a sealed space which is filled with a liquid crystal material. A transparent front electrode and an array of back electrodes are disposed on opposite sides of the space and are selectively energized to apply electric fields to the liquid crystal material to cause it to locally change its orientation resulting in a spatially variant perturbation of the light passing through. A capacitor is connected in parallel with each back electrode for charge storage.
Different liquid crystal materials affect light passing therethrough by different mechanisms, such as variable birefringence, scattering, etc. The display may provide only discrete black and white levels, or a continuous gray scale. In either case, the liquid crystal material will appear dark when polarized such that it is clear and incident light is reflected away from the viewer by the back electrodes, and will appear light when polarized such that it is opaque and scatters the incident light.
The back electrodes can be arranged in segments to provide an alphanumeric display for a calculator or clock, or in a rectangular matrix to provide a continuous graphic image for television, computer and other applications.
U.S. Pat. No. 4,239,346, entitled "COMPACT LIQUID CRYSTAL DISPLAY SYSTEM", issued Dec. 16, 1980 to R. Lloyd discloses a reflective Active-Matrix Liquid-Crystal Display (AMLCD) including a transparent front electrode and back electrodes which define a sealed space therebetween which is filled with liquid crystal material. The front electrode is formed on the inner surface of a front plate, whereas a semiconductor layer and back electrodes are formed on the inner surface of a back plate.
The back electrodes are formed of a reflective metal in a rectangular matrix pattern on the inner surface of the semiconductor layer in contact with the liquid crystal. MOSFET electrode driver transistors, interconnected by polycrystalline silicon bus lines, are also formed in the semiconductor layer, and are operatively connected to the electrodes. Electrical potentials are selectively applied between the individual back electrodes and the front electrode via the bus lines and driver transistors to locally polarize the liquid crystal material and form an image complete with gray scale.
The preferred material for the front and back plates is glass, due to its negligible reactivity with liquid crystal materials, low cost and transparency. Although epitaxial deposition of monocrystalline (single crystalline or bulk) silicon is possible on various materials such as sapphire, the temperature required for deposition is on the order of 1,000.degree. C., which is far in excess of the melting point of glass. In addition, the atomic structure of glass is highly irregular, and non-conducive to the growth of an epitaxial silicon layer.
For these reasons, the silicon layer on the back plate of Lloyd's display is formed by a standard silicon wafer. Alternative displays have used silicon layers formed by chemical vapor deposition (CVD) of amorphous or polycrystalline silicon. CVD of these materials can be performed at low temperatures and is not adversely affected by the irregular crystalline structure of the glass material of the plate.
However, the carrier mobility of polycrystalline silicon is one-eighth that of monocrystalline silicon, and the mobility of amorphous silicon is one-hundredth that of monocrystalline silicon. The operating speed of a microelectronic device is linearly proportional to the mobility. The low mobility of polycrystalline and amorphous silicon limits the operating speed of the electrode driver transistors and thereby the displays in which they are incorporated. These devices generally operate at less than 60 Hz, which is a common video display speed. In order to accommodate the leakage current and refresh time of transistors fabricated in polysilicon for 60 Hz operation, two transistors are connected in series to obtain sufficiently high impedance and low current.